In one form of magnetic storage, bits of encoded digital data are stored as magnetized portions of a magnetic medium. This medium is supported on a surface which is provided, for example, in the form of an elongated tape, a drum or a disk. In accordance with a selected coding technique, each bit of digital data may be represented by a corresponding transition from a first signal level to a second signal level, as for example in phase coding. Alternatively only digital "ONES" may be represented by a corresponding transition from a first signal level to a second signal level, as for example in NRZI coding. Recording of digital data is performed by establishing relative motion between a recording head and the medium and then exciting the recording head in accordance with the selected coding technique to produce flux transitions within the recording head. Each flux transition is produced by a transition or step in the amplitude of the current flowing in a winding mounted on the recording head. The medium adjacent the recording head stores the encoded digital data in the form of magnetized portions produced in response to the flux transitions.
Recovery of stored data is accomplished by establishing relative motion between the magnetic medium and a readout head thereby producing a corresponding readout pulse for each flux transition which occurred during recording of the digital data.
U.S. Pat. No. 3,503,059 entitled "Pulse Coding Compensation for Magnetic Recording" issued Mar. 24, 1970 in the name of L. E. Ambrico discloses an apparatus and a method to compensate and effectively eliminate peak shift and improve the amplitude response of the readout head. Peak shift is the displacement in position with respect to time of the peak amplitude of a readout pulse. Peak shift is a function of the relative spacing between flux transitions during recording of the digital data. If the spacing between transitions is relatively large then the peak amplitude has been observed to be correctly positioned. FIG. 1 illustrates that the flux transition designated A produces a pulse A' at the readout head. The pulse A' has its amplitude correctly positioned but is asymmetrical because the fall time (T.sub.f) of the pulse is longer than the rise time (T.sub.r). This effect has been commonly observed in recording systems and is caused by the fact that a step of write current produces a distorted step of magnetization in the magnetic medium.
FIG. 2 shows that the flux transitions B and C would produce the readout pulses B' and C' if the spacing between the flux transitions were large. Each pulse would have its respective peak amplitude correctly positioned but would be asymmetrical because the trailing edge would be longer than the leading edge. However, since the flux transitions are spaced relatively close to each other, the actual pulses produced at the readout head are B" and C" constituting the linear superposition of B' and C', this effect being well established in the magnetic recording art. It has been observed that when the two flux transitions are positioned relatively close to each other, the peak of the first pulse is reduced in amplitude (.DELTA.P.sub.B) and shifted forward in time (.DELTA.T.sub.B) with respect to its correct position and the peak of the second pulse is more reduced in amplitude (.DELTA.P.sub.c) and shifted backward in time (.DELTA.T.sub.c) with respect to its correct position. With respect to the asymmetry of readout pulses, the interference of the leading edge of C' on the trailing edge of B' is less than the effect of the trailing edge of B' on the leading edge of C' because the rise time of C' is faster than the fall time of B'. It follows that a significant improvement in the symmetry of the readout pulses will result from decreasing the fall time of B'.
As taught in the aforementioned patent of Ambrico the fall time of the readout pulse may be substantially decreased by providing a minor flux transition of lower amplitude and opposite direction following each major flux transition. While the teachings of Ambrico have enabled readout pulses to be provided which are more symmetrical and narrower than readout pulses without such compensation, it has been observed that the effect of such compensation is limited by the unimproved leading edge of the readout pulse.
With further reference to FIG. 2, utilizing the compensation taught by Ambrico will produce a readout pulse B" in which the trailing edge has returned to the base line sooner than the trailing edge of a readout pulse without such compensation. However, in most practical systems some interference will remain because there has been no improvement in the long leading edge of C'. Thus the teachings of Ambrico are limited in the results obtainable by the rise time of the readout pulse. In addition, if the compensation of Ambrico is optimized to shorten the fall time of the readout pulse to be less than its rise time, asymmetry will again be produced. The packing density is also limited by the pulse width, therefore, any increase in packing density is also limited by the rise time of the readout pulse. It follows that the prior art including the compensation taught by Ambrico is limited in the achievable packing densities by the configuration of the readout pulse. Although the readout pulses may be further narrowed by employing the compensation taught by Ambrico, some interference will still remain because of the long leading edge of the succeeding pulse.
FIG. 3 shows the effect of interference between three equally spaced relatively close flux transitions occurring in the encoded digital data. The peak of the first pulse is reduced in amplitude (.DELTA.P.sub.D) and shifted forward in time (.DELTA.T.sub.D) with respect to its correct position and the peak of the third pulse is also reduced in amplitude (.DELTA.P.sub.F) and shifted backward in time (.DELTA.T.sub.F) with respect to its correct position. However, while the peak of the second pulse is correctly positioned in time because it is subjected to equal forward and reverse shifts, it is substantially reduced in amplitude (.DELTA.P.sub.E). As a result, the interaction of these pulses significantly effects the accuracy of the detected digital data and limits the amount of data that may be recorded on the storage medium.